In a high intensity discharge (HID) lamp, a medium to high pressure ionizable gas, such as mercury or sodium vapor, emits visible radiation upon excitation typically caused by passage of current through the gas. One class of HID lamps comprises electrodeless lamps which generate an arc discharge by generating a solenoidal electric field in a high-pressure gaseous lamp fill. In particular, the lamp fill, or discharge plasma, is excited by radio frequency (RF) current in an excitation coil surrounding an arc tube. The arc tube and excitation coil assembly acts essentially as a transformer which couples RF energy to the plasma. That is, the excitation coil acts as a primary coil, and the plasma functions as a single-turn secondary. RF current in the excitation coil produces a time-varying magnetic field, in turn creating an electric field in the plasma which closes completely upon itself, i.e., a solenoidal electric field. Current flows as a result of this electric field, resulting in a toroidal arc discharge in the arc tube.
At room temperature, the solenoidal electric field produced by the excitation coil is typically not high enough to ionize the gaseous fill and thus initiate the arc discharge. One way to overcome this shortcoming is to lower the gas pressure of the fill, for example, by first immersing the arc tube in liquid nitrogen so that the gas temperature is decreased to a very low value and then allowing the gas temperature to increase. As the temperature rises, an optimum gas density is eventually reached for ionization, or breakdown, of the fill to occur so that an arc discharge is initiated. However, the liquid nitrogen method of initiating an arc discharge is not practical for widespread commercial use.
More recent methods for starting electrodeless HID lamps entail using starting aids to capacitively couple the high voltage developed across the excitation coil turns into the arc tube. As a result of this voltage gradient, a capacitive current flows between the starting aid and the excitation coil, and hence through the arc tube, thereby ionizing the gaseous fill and producing a low current glow discharge therein. When the gas is sufficiently ionized, a transition is made from a relatively low current glow discharge to a relatively high current, high intensity solenoidal arc discharge. Such a starting aid may comprise, for example, a pair of capacitive starting electrodes, as described in U.S. Pat. No. 4,902,937 of H. L. Witting, issued Feb. 20, 1990, and assigned to the instant assignee, which patent is incorporated by reference herein. Each starting electrode comprises a conductive ring which surrounds the arc tube and is connected to the excitation coil of the HID lamp. Coupling a high voltage signal between the pair of starting electrodes causes an electric field to be produced therebetween which is of sufficient magnitude to create a glow discharge in the arc tube due to the arc tube wall capacitance. Furthermore, as it has been determined that the application of relatively large electric fields directly to the arc tube via the starting aid may cause early arc tube degradation, heat sensitive members, e.g. bimetallic strips, are utilized for moving the starting electrodes away from the arc tube after initiating an arc discharge, thereby preserving the useful life of the lamp.
Recently developed fixed starting aids for electrodeless HID lamps include means for removing the starting voltage from a fixed starting electrode after initiation of the arc discharge. For example, a gas probe starter includes a fixed starting electrode coupled to a starting chamber containing a gas, preferably at a low pressure relative to that of the arc tube fill, which chamber is attached to the outer surface of the arc tube. One such starting aid is the subject of commonly assigned Roberts et al. U.S. patent application, Ser. No. 622,247 (docket No. RD-19,981), filed Dec. 4, 1990, which is incorporated by reference herein. As described in the Roberts et al. application, a starting voltage is applied to the starting electrode by a starting circuit, causing the low-pressure gas in the starting chamber to become conductive. As a result, a sufficiently high voltage is applied to the arc tube to ionize the gaseous fill in the arc tube, thus forming an arc discharge therein. After the lamp has started, the starting voltage is removed from the starting electrode in order to extinguish the discharge current in the chamber which would otherwise have a detrimental effect on the arc tube wall.
A suitable starting circuit for applying a starting voltage to either a fixed or movable starting aid is described in commonly assigned, copending U.S. patent application Ser. No. 527,500 of S. A. El-Hamamsy and R. J. Thomas, filed May 23, 1990, which is incorporated by reference herein. The starting circuit of the El-Hamamsy and Thomas application comprises a high-efficiency power supply, including a tuned output circuit, for providing an RF signal to the starting electrode. Exemplary high-efficiency power supplies useful in the El-Hamamsy and Thomas starting circuit comprise Class-D and Class-E power amplifiers. The starting circuit operates at a higher frequency than that used to operate the lamp ballast which drives the excitation coil. In this way, the starting circuit is independent of the lamp ballast circuit and, therefore, does not interfere with the operation thereof.
Another suitable starting circuit is described in J. P. Cocoma and G. A. Farrall U.S. patent application No. 622,024 (docket No. RD-20,228), filed Dec. 4, 1990, which is assigned to the instant assignee and incorporated by reference herein. Cocoma and Farrall describe a passive starting circuit wherein a series resonant circuit is tuned to provide a starting voltage substantially simultaneously with the application of power to the excitation coil from the main power source.
One shortcoming of passive starting circuits is that stray capacitance effects can cause detuning, resulting in less than optimum performance. Furthermore, the reliance on stray capacitances to tune the circuit implies that even slight layout changes (e.g., the position of the coil in the circuit) cause significant detuning of the starting circuit. Therefore, it is desirable to avoid such stray capacitance effects and thus optimize starting circuit performance, i.e. enhance starting reliability.